We could certainly DM an invoice - that’s easy to do. How should a user signal they want an invoice from within their nostr client? 

We could make it a chat bot with commands like nostr:npub1fyvwkve2gxm3h2d8fvwuvsnkell4jtj4zpae8w4w8zhn2g89t96s0tsfuk but that seemed a bit archaic to me.